Error determination for multi-axis apparatus due to thermal distortion

Abstract

Apparatus with a multi-axis mechanism to perform, say, measurements or tests, has at least one pair of temperature detectors individually for each of selected members of the mechanism, each pair of temperature detectors being arranged to sense temperature differences transversely across the associated member. The required positions of the temperature detectors are predetermined from a preliminary examination of the mechanism, so that the extent and direction of each predetermined aspect of thermal distortion within the mechanism, can be computed from simultaneously detected transverse temperature differences, employing algorithms devised as a result of the preliminary examination of the mechanism. Compensation for corresponding selected errors in the measurements or tests can be calculated from the computed extent and direction of each of the predetermined aspects of thermal distortion within the mechanism.

We claim: 
     
       1. Apparatus with a mechanism to perform movements in relation to a plurality of axes associated with the mechanism, comprising means to compute the extent and direction of each predetermined aspect of thermal distortion capable of occurring within the mechanism due to temperature differences transversely across each of a selected plurality of constituent members of the mechanism, said means including at least one pair of temperature detectors provided individually for each of the selected plurality of members, each pair of temperature detectors being arranged to sense differences in temperature in relation either to a direction transversal to an axis of the member or to one of two directions transversal to said axis and wherein said two directions are at an angle to each other. 
     
     
       2. Apparatus as claimed in claim 1, in which at least one of the selected members of the mechanism comprises a beam member. 
     
     
       3. Apparatus as claimed in claim 2, having a measuring or testing mechanism including a work table on which a work piece to be measured or tested is to be positioned, a bridge having a beam mounted at its ends on pillars and spanning the work table, the bridge being free to move relative to the work table, a probe carrier mounted on the bridge beam, and being free to move along the bridge beam and a probe tip at the end of a column, the column being supported by the probe carrier and being free to move along its longitudinal axis relative to the probe carrier, said plurality of selected members within the mechanism including one pillar for the bridge beam, the bridge beam, and the column having the probe tip at one end. 
     
     
       4. Apparatus as claimed in claim 1 in which the temperature detectors comprise electrical temperature detectors. 
     
     
       5. Apparatus as claimed in claim 1 in which there is a plurality of pairs of temperature detectors in relation to a transverse direction across at least one selected member, the plurality of pairs of temperature detectors being spaced from each other along the longitudinal axis of the member. 
     
     
       6. Apparatus as claimed in claim 5 in which the plurality of pairs of temperature detectors are connected in series. 
     
     
       7. Apparatus as claimed in claim 5 in which the arrangement is such that there is to be detected any temperature differences in relation to said transverse direction across the member, individually, for a plurality of constituent sections of the member, the sections being distributed along the longitudinal axis of the member. 
     
     
       8. Apparatus as claimed in claim 1 in which at least one of the selected members of the mechanism comprises a beam member having a hollow form of construction, and each associated pair of temperature detectors is positioned within the hollow beam member. 
     
     
       9. Apparatus as claimed in claim 8 wherein the temperature detectors are electrical and further include leads extending from the detectors along the longitudinal axis of the beam member to a point external of the beam member. 
     
     
       10. Apparatus as claimed in claim 1, in which at least one pair of temperature detectors, comprises electrical temperature detectors in the form of a pair of thermocouple junctions arranged back-to-back with a length of one thermocouple material extending between the two junctions and two leads of another thermocouple material extending individually from the two junctions. 
     
     
       11. Apparatus as claimed in claim 1 in which computed errors within the mechanism, comprising the extent and direction of each predetermined aspect of thermal distortion within the mechanism, or corresponding derived errors in movements calculated from the computed errors due to thermal distortion within the mechanism, are included within an active error map. 
     
     
       12. Apparatus as claimed in claim 11, in which computed errors within the mechanism, comprising the extent and direction of each of the predetermined aspects of thermal distortion within the mechanism, or corresponding derived errors in movements calculated from the computed errors due to thermal distortion within the mechanism, are superimposed on a geometric error map.